1. Field of the Invention
The present invention relates to an electrically driven vibratory conveyor.
2. Description of the Prior Art
Vibratory conveyors comprise a stationary frame such as a base plate, on which springs are mounted. The springs support a conveyor element such as a cup, a plate or the like. The springs and the conveyor element form a vibratable mass-spring-system which may be electromagnetically driven to oscillate. For this purpose, at least one soft iron yoke carrying at least one coil is mounted on the frame while the conveyor element carries a soft iron armature defining with the yoke an air gap. If the coil or coils are excited with AC current the conveyor element will oscillate in synchronism with the AC frequency, in particular with 100 oscillations per second on a mains frequency of 50 Hz.
The exciting frequency will rarely coincide with the resonant frequency of the mass-spring-system although this would result in the configuration of least energy consumption. Moreover, the resonant frequency of the mechanical system is not constant but depends upon the (additional) mass of the conveyed parts.
It is evident that a sensor may be mounted on the mechanical system so as to monitor the actual oscillation frequency and to feed it back to an amplifier which delivers the exciting currents for the coil or coils. Such conveyors are disclosed for example in German published patent applications DE-A-41 42 398 and DE-A-36 44 811.
Conveyors of the type defined above, however, are not provided with such pickup systems and would have to be modified if the user wanted to operate them in the resonant mode.
It is therefore an object of the present invention to provide a driver unit for electromagnetically vibratable conveyors to be driven at least close to the resonant mode and wherein no modifications of the mechanical system need be provided.